Preparation of an alumina catalyst carrier from alumina generally uses a step of kneading an alumina material, a step of forming the kneaded material into a desired shape (such as cylinder, pellet or honeycomb) and a desired size, a step of drying the formed product at a temperature of 80 to 150.degree. C., and a step of calcining the dried product in a calciner at a calcination temperature of 400 to 1,000.degree. C.
Such an alumina catalyst carrier is required to have high activity and high strength. It is difficult to fulfill both of these requirements, and attempts have been made to do so. Japanese Laid-Open Patent Publication No. 8445/91 of the applicant, for example, discloses a process for preparing an alumina carrier which comprises adding an acid to a slurry consisting of an alumina hydrate and water to make pH 2.0-3.0, then adding an alkali to adjust the pH of the slurry to 3.5-6.0, and then separating an alumina hydrogel prior to the kneading of a starting alumina powder. The use of this method makes it possible to adjust the pore diameters of an alumina carrier, thereby improving the activity of a catalyst.
Japanese Laid-Open Patent Publication No. 235737/92 discloses a method comprising pressure molding water-containing alumina, followed by an aging and calcining process. This method is described as being capable of obtaining an alumina carrier having pores of 1,000 angstroms or more in diameter and having high mechanical strength. This publication, moreover, discloses a method which involves impregnating the alumina carrier with an aqueous solution of an aluminum salt, followed by a drying and calcining process, to further enhance its mechanical strength.
Japanese Laid-Open Patent Publication No. 98486/75 discloses a process for preparing a high-strength, high-activity alumina catalyst carrier by adding an acidic substance, such as acetic acid, and water to alumina, kneading them, forming the mixture, and calcining the formed product.
Japanese Laid-Open Patent Publication No. 27436/92 discloses a hydrogenation catalyst using an alumina catalyst in which the lengths of alumina particles after forming and calcination steps have a predetermined distribution. This publication also discloses a process for preparing the hydrogenation catalyst. This process is described as being excellent in desulfurization activity.
Japanese Laid-Open Patent Publication No. 205990/94 discloses the production of a high-activity catalyst for the hydrogenation and desulfurization of a metal-containing heavy oil by use of an alumina carrier having a specific pore diameter distribution. In Example A of this publication, an alumina material comprising two kinds of alumina particles, Catapal alumina and Versal 250 alumina is used for the preparation of a catalyst carrier. No mention is made, however, of the aspect ratios of these alumina particles.
An alumina catalyst carrier is required to have pore diameters of about several tens of nanometers and a narrow pore diameter distribution in order to achieve improved catalytic activity. To produce a catalyst carrier having such characteristics, it is desirable to use the starting alumina particles with an aspect ratio of 5 or less. The catalyst carrier, however, should also have a large specific surface area and pore volume to have high catalytic activity, thus making it impossible to proceed fully with the sintering of the particles. Thus, when particles with an aspect ratio of 1 to 5 are used as an alumina material, such particles can be joined to each other only at the site of contact between the particles at the time of calcination. The resulting catalyst carrier is so low in mechanical strength as to undergo chipping or dusting.
When fibrous particles having an aspect ratio of greater than 10 are used as the starting material, on the other hand, the fibrous particles are entangled during kneading, thus increasing the strength of the resulting catalyst carrier. This effect increases as the aspect ratio becomes higher. The carrier using such a starting material is low in catalytic activity because of a broad pore diameter distribution.
The object of the present invention is to provide a process for preparing an alumina catalyst carrier having sufficient mechanical strength, being capable of imparting high catalytic activity, and minimally undergoing chipping or dusting.